crane: add --flatten for index append

[jonjohnsonjr]

While it's possible that folks might want to append an index to an index, it's more likely that they would want to merge them. This flag gives them an option to do either.

[tianon]

I've tested this successfully with a list of images that contains both indexes and a non-index manifest and it flattened correctly 👀

[codecov-commenter]

Codecov Report

Merging #1566 (72bade8) into main (3624968) will not change coverage.
The diff coverage is n/a.

❗ Current head 72bade8 differs from pull request most recent head 72dda39. Consider uploading reports for the commit 72dda39 to get more accurate results

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  Hits         6870     6870           
  Misses       1855     1855           
  Partials      692      692           

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